Hot Ice

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Chemistry, Earth Science: Super Saturated Solutions, Crystal Formation
Grade Range: Elementary School, Middle School, High School
Format: Hands-on, Stage

This demonstration is easy to set up and easy to present. This demonstration can be redone every 15-20 minutes, and after preparing the solution it can be reused for months at a time.

Contents

Materials

  • Sodium Acetate
  • Sodium Acetate Solution (in 250mL Erlenmeyer Flask)
  • Ice Water
  • Hot Plate
  • Watch Glass or Similar

Safety Precautions

Please read the Liquid Chemical section of the Demonstration Safety page before performing this demonstration.

This demonstration requires: Goggles, gloves (during preparation)

Demonstration

Preparation

Prepare the Sodium Acetate Solution: measure out 65mL of the sodium acetate and pour into the volumetric flask. Add water to fill it to the 200mL line, and heat the solution while stirring until all the crystals dissolve into the solution, about 30 minutes. If there is trouble with getting all the crystals to dissolve, then add water in small 3-5mL increments. Once all the crystals have dissolved, add a small pinch more of the crystals and dissolve them into it. Remove the solution from heat and cork it, and allow the solution to cool slowly to room temperature. Keep stored in a chemical cabinet and label it with the date it was prepared.

Presentation (Stage)

Prior to presenting, if the solution is crystallized then reheat it on the hot plate for 10-15 minutes to remelt it. Set the solution carefully into a bowl with ice water to cool it. It can be kept in the ice water at the start of the show.

  1. Ask the audience of any of them have ever seen stalagmites or stalactites in person. Can anyone guess how they form?
  2. Show the flask and mention that the solution inside is a super saturated solution. Set out the watch glass with a small seed crystal in the center, and ask the audience What they think might happen if if poured out the solution.
  3. Carefully start pouring the solution directly on top of the seed crystal. You should see a stalagmite start to form! Try to pour out the entire solution and create a large stalagmite. The solution is very sensitive, so it might crystallize inside the flask as you are pouring!
  4. Alert the audience that not only did the solution crystallize, but it also heated up as it did so. Why would it heat up? Why did it crystallize?
Presentation (Hands-on)

Prior to presenting, if the solution is crystallized then reheat it on the hot plate for 10-15 minutes to remelt it. Set the solution carefully into a bowl with ice water to cool it. It can be kept in the ice water at the start of the show.

  1. Show the students the flask, and ask them what the solution inside looks like (most will say water, they might note some slight discoloration). Let a student feel the flask and note that it is currently cold, like the ice water.
  2. Shake the solution and set it down quickly onto the table. Tell the students to watch the solution. They will see small snowflake crystals form, and from there the whole solution will crystallize in under a minute!
  3. Ask the same student from before to feel the outside of the container, and ask them if the flask is now warmer or cooler. Why is it warmer? What caused the solution to crystallize?

Why This Works

Super Saturated Solutions are solutions that contain more of a dissolved compound than they would normally be able to contain. This often happens when a solvent, like water, is first heated before a solute, like our sodium acetate, is added to it. If you add in enough of the solute until the solvent almost cannot contain more of it, and the solution is then allowed to cool while undisturbed, then the solute will stay in solution. The solution, however, has a lot of stored energy due to the excess solute, and it wants to release that energy. In order to release it, however, it needs to have an Activation Energy, or energy come in from an outside source in order to kick-start the process. Along with the activation energy, the solution might require a seed crystal, or a small crystal to provide a starting point for the crystallization process.

When the solution is poured out onto the watch glass, the small amount of energy given from being poured out, along with the seed crystal on the watch glass, is enough to trigger the crystallization. The solution can also start to crystallize inside the flask if there is enough activation energy, such as when the bottle is shaken. Lastly, the solution can crystallize if any of the solvent were to evaporate away and cause the concentration to be too high to stay in solution, which is why the solution crystallizes on its own if left open for extended periods!

In a cave or cavern, stalagmites and stalactites can form due to minerals coming out of the water in the cave. The water in a cave can have very high concentrations of various minerals, to the point of supersaturation. As water drips from a cave ceiling, it may leave behind a small mineral crystal on the ceiling. Over time, the mineral deposits can build on top of each other, and eventually form a stalactite. Likewise, as water drips onto the floor of the cave, the impact with the floor may cause a mineral to crystallize out of solution. Over time, these drops can build on top of each other to create a stalagmite!

Additional Information

  • Supersaturated solutions of sodium acetate are commonly used in reusable hot packs! The packs will have a small metal disc inside that you can snap to act as a seed crystal, and to reuse them you simply reheat them in hot water and let them cool.
  • This demonstration is a part of the Fire and Ice Show
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